It is a long-standing objective of color photographic origination materials to maximize the overall response to light while maintaining the lowest possible granularity. Increased photographic sensitivity to light (commonly referred to as photographic speed) allows for improved images captured under low light conditions or improved details in the shadowed regions of the image. In general, the overall light sensitivity provided by the light sensitive silver halide emulsions in such systems is determined by the grain size of the emulsions. Larger emulsions capture more light. In color photographic elements, upon development, the captured light is ultimately converted into dye deposits which constitute the reproduced image. However, the granularity expressed by these dye deposits is directly proportional to the grain size of the silver halide emulsion. Thus, larger silver halide emulsion grains have higher sensitivity to light but also lead to higher granularity in the reproduced image. It has been a long-standing problem to provide materials which maximize the response to light of a silver halide emulsion for any given grain size.
3D, core/shell bromide emulsions containing high iodide regions have long been a staple of the blue-sensitive layer in color film. Their intrinsic light absorption in the blue region together with their low response to pressure, continue to make them an attractive choice, especially as the fast component. Recent techniques have been developed to improve the photographic performance of such emulsions by introducing twin planes (Matemaghan in U.S. Pat. No. 4,184,877), producing grains with a particular iodide architecture (Takada et al in U.S. Pat. No. 4,668,614, Ishikawa et al in U.S. Pat. No. 4,963,467), narrowing the range of iodide in individual grains (Shibahara et al in U.S. Pat. No. 4,728,602), and growing grains free of renucleation while obtaining a narrow distribution of grains with a high iodide content (Chang et al, U.S. Pat. No. 5,570,327). Although these techniques have, indeed, increased performance of core/shell emulsions there continues to be a need for further improvements to yield color film with the highest possible image quality for the consumer.
It is of particular interest to find solutions to this problem for large emulsions with the potential for providing high speed (preferably ISO 400 or greater) color photographic materials. Such high-speed materials have a number of potential applications. They are particularly valuable for use in cameras with zoom lenses and in single use cameras (also called "film with lens" units). Zoom lenses generally are limited to smaller apertures than non-zoom lenses, which reduces light intensity. Thus, zoom lenses, while giving increased flexibility in composition of a pictorial scene, deliver less light to the camera film plane. Use of high-speed films allows the flexibility of zoom lenses while still preserving picture-taking opportunities at low light levels. In single use cameras, lens focus is fixed. Here, high-speed films allow use of a fixed aperture having a higher f-number, thus increasing the available depth of field, an important feature in a fixed focus camera. For single use cameras with flash, higher film speed allows pictures to be taken with a less energetic flash, enabling more economical manufacture of the single use unit. The introduction of the Advanced Photo System has further increased demand on film image quality by reducing camera size and, concomitantly, the size of the image-capturing element.
3D, core/shell emulsions, while capable of the highest speeds of any emulsion type in the blue record, have the particular disadvantage of producing a relatively low contrast where contrast is defined as the slope or gradient of the linear portion of the density vs. log exposure or D-Log E curve. The low contrast originates chiefly from two sources: the relatively wide dispersity in grain size characteristic of large, grains and the high iodide content of the grains. Both of these features, i.e., large grain size and high iodide content are required to obtain the greatest possible blue speed and, therefore, are inherent in this type of emulsion. A need, thus, exists for an additional and independent technique for improving performance.